Research Article pubs.acs.org/journal/ascecg
Fabrication of 3D Macroscopic Graphene Oxide Composites Supported by Montmorillonite for Efficient U(VI) Wastewater Purification Wencai Cheng,†,‡ Congcong Ding,*,† Xiaoqin Nie,*,† Tao Duan,† and Ruirui Ding§ †
School of National Defence Science & Technology and ‡Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory, Southwest University of Science and Technology,Mianyang 621010, PR China § Shanxi Provincial TCM Hospital, Taiyuan 030012, China S Supporting Information *
ABSTRACT: In this work, montmorillonite (MMT) was intercalated into a sheet of GO and then cross-linked by agar, realizing the 3D macroscopic composites of agar-MMT-GO. Agar-MMT-GO composites were characterized by SEM, TEM, FTIR, TGA, and XPS techniques. Fabricated agar-MMT-GO composites were tough and lightweight materials with a multichannel structure. The heat stability of agar-MMT-GO composites were enhanced relative to GO, as demonstrated by TGA results. FTIR analysis testified to no chemical bond between GO and MMT component, suggesting that agar scaffold mainly caused the formation of agar-MMT-GO composites. AgarMMT-GO composites exhibited a good performance in U(VI) sorption (Qmax of 147 mg/g at pH 4.5) and in recycling tests. The rate-limiting sorption step was the diffusion of U(VI) from liquid solution to the surface sites of agar-MMT-GO composites. XPS analysis demonstrated that U(VI) sorption on agar-MMT-GO was mainly attributed to cation exchange and inner-sphere complexation in low and high pH regions, respectively. Briefly, our work here provides new insights in designing GO composites with a solid skeleton to promote its large-scale application in wastewater remediation. KEYWORDS: Hybrid composites, U(VI), Graphene oxide, Agar-MMT-GO, Removal, Recycle
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INTRODUCTION Graphene oxide (GO) nanosheets offer great efficiency in water treatment due to their extraordinary hydrophilic, high surface area to mass ratio and easy incorporation of specific functionality.1−4 Liu et al.5,6 found that a higher oxidation degree of GO resulted in a higher radionuclide sorption capacity. The introduction of functional groups (i.e., phosphate and PANI) on GO can enhance its high sorption performance and achieve selective extraction of U(VI) from environmental pollutants.7−12 Although GO nanosheets have exhibited promising potential in treating heavy metals pollution,13−19 their application was limited to experimental tests because GO nanosheets were used in the form of powder. Consumption of energy and time is needed for separation and recovery of GO nanosheets via high-speed centrifugation or filtration after the treatment. Moreover, the risk of GO as a nanomaterial to aquatic environment should not be ignored. The adverse impacts of GO on aquatic organisms have been reported.20 Assembling GO nanosheets to macroscopic devices can be a strategy to resolve this problem, thus developing cost-effective water treatment processes, yet as well as we are aware, only a few reports focus on assembling of GO nanosheets into macrosized bulk nanocomposites.21−24 The bottleneck of this © 2017 American Chemical Society
issue was getting the high sorption property and tough structure together. Some researchers used a suction filtration method to get 3D GO,25,26 although the obtained materials by this method showed some defects: low reaction rate and easy dispersion in solutions. Thus, there is demand for macromolecular framework materials to help fabricating 3D materials,27,28 where the great challenge is to form a loose pore structure. This study used agar, a common biological macromolecules with super cross-linking properties,29 as a kind of loose pore maker. Agar has been extensively used in food, medicine, chemical industry, textile, national defense, and other fields.30 Its solution easily formed a gel upon cooling down to room temperature, which would create a porous scaffold structure.31 This kind of structure had many advantages, like high hydrophilicity, good support strength, great tenacity, and so on.32 The mixtures of GO and agar were easily collapsed because of the principle of similar compatibility in our preliminary test. To overcome this shortcoming in the binary Received: March 19, 2017 Revised: April 11, 2017 Published: April 24, 2017 5503
DOI: 10.1021/acssuschemeng.7b00841 ACS Sustainable Chem. Eng. 2017, 5, 5503−5511
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Principle diagram of synthesis process (a), SEM pictures of GO (b) and MMT (c), digital camera photographs of agar-MMT-GO composites (d, e). 1 h and ultrasonication for 0.5 h to obtain the homogeneous suspensions, respectively. The agar was injected into the homogeneous suspensions. The agar-MMT-GO composites supported by agar were obtained via vacuum freeze-drying. Characterization. SEM images were obtained by using a fieldemission scanning electron microscope (JEOL-7500F). TEM images were obtained by using a FEI Tecnai G20 instrument at 200 kV. The internal and interlayer profiles of U(VI)-loaded agar-MMT-GO composites were analyzed by TEM images with elemental mapping. FTIR spectra were conducted at an iN10MX FTIR instrument in the range of 4000−400 cm−1 using the KBr disc technique. TGA was performed on a NETZSCH TG 209F1 Libra under argon with a temperature increase rate of 20 °C per min. XPS was performed for original and U(VI)-loaded (pH 3.0 and 10.0) samples using an ESCALab220i-XL (Thermo Scientic) with the X-ray source of monochromatic Al Kα radiation at 150 W. The high-resolution data was collected using a pass-energy of 20.0 eV with a step size of 0.1 eV. The energies were corrected using the C 1s peak at 284.6 eV as a reference, and the data were analyzed using the XPSPEAK software (version 4.1). Experimental conditions for XPS sample preparation adhered to the following sorption experiment. Batch Sorption Experiment. Batch sorption experiment was conducted with 1 g/L agar-MMT-GO and 15 mg/L U(VI) solutions at T = 303 K in the presence of 0.001 mol/L NaClO4 solutions. The pH of the system was adjusted to 4.5 to prevent U(VI) precipitation. Briefly, the bulk suspensions of agar-MMT-GO and NaClO4 were preequilibrated for 24 h in closed polycarbonate tubes, and then the U(VI) solution was added into the tubes. Negligible volumes of 0.1− 1.0 mol/L NaClO4 and NaOH were used to achieve the investigated pH range from 3.0 to 9.0. The ionic strength factors were manipulated in 0.001, 0.01, and 0.1 mol/L NaClO4 gradient solutions at T = 303 K. Sorption isotherms of U(VI) were investigated with U(VI) concentration ranging from 5 to 40 mg/L in 0.001 mol/L NaClO4 solution at pH 4.5 and T = 303, 313, and 323 K. On the basis of the kinetics results, 24 h of shaking time was enough to guarantee reaction equilibrium. After 24 h, the concentrations of U(VI) in the supernatant were measured using spectrophotometrically Arsenazo III method at wavelength of 650 nm. All experimental data were the average of triplicate determinations. The percentages and capacities of U(VI) on agar-MMT-GO (adsorption (%) and q (mg/g), respectively) were determined according to eqs 1 and 2, respectively.
system, we used montmorillonite [MMT, (Al, Mg)2[Si4O10](OH)2·nH2O)] as a filler support matter. MMT is an economical material with high ion exchange capacity and sorption ability.33,34 In this study, the ternary agar-MMT-GO nanocomposites were manufactured based on the synergistic effects of GO, MMT, and agar emerging from building blocks and interfacial interactions. Agar-MMT-GO nanocomposites not only possessed the tough mechanical property and high sorption ability of each component but also extended the bulk scale of GO to macroscale level, avoiding the pollution from GO and facilitates the separation from wastewater. The purposes of this study are (1) to successfully synthesize and characterize agar-MMT-GO nanocomposites using scanning electron microscopy (SEM), transmission electron microscope (TEM), thermal gravimetric analysis (TGA), and Fourier transform infrared spectra (FITR); (2) to investigate the adsorption of U(VI) on agar-MMT-GO composites under various environmental conditions (i.e., contact time, pH, and temperature) by batch techniques; and (3) to discuss the interaction mechanism between U(VI) and agar-MMT-GO composites by using X-ray photoelectron spectroscopy (XPS). Our findings provide an innovative insight into the design and preparation of robust nanocomposite hydrogel on a macroscale.
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EXPERIMENTAL SECTION
Materials. Flake graphite (20 μm, 99.95% purity) was purchased from Qingdao Tianhe Graphite Co. Ltd., China. Na+-type MMT was purchased from Zhejiang Fenghong Clay Co. Ltd. MMT was treated with 0.5 mol/L HCl solution, then washed by deionized water to neutral pH, and finally dried via lyophilization. Appropriate HCl treatment can eliminate the impurities and modify MMT surface area to enhance its proton exchange ability. Concentrated sulfuric acid (98 wt %), hydrochloric acid, sodium nitrate, agar, potassium permanganate, hydrogen peroxide, and ethanol were of reagent grade and were purchased from Beijing Chemical Works Co., Ltd. U(VI) stock solution (1.0 g/L) was prepared from UO2(NO3)2·6H2O in a 0.01 M HNO3 solution. All the reagents were used without further purification. Fabrication of Agar-MMT-GO Nanocomposites. GO was prepared by the modified Hummers’ method.35 More details can be found in our previous paper.17 A 3 g amount of MMT was dispersed into 100 mL of 0.1 mol/L HCl solution and stirred for 3 days; then, the suspensions were centrifuged at 3000 rpm for 15 min. The collected MMT was dispersed to deionized water to get a concentration of 2.8 mg/mL. GO suspension (1.0 mg/mL) was prepared by adding GO into deionized water and stirring for 1 h, and then subjecting the solution to ultrasonication for 0.5 h. As-obtained GO suspensions were mixed with the MMT suspensions. Subsequently, the mixing suspensions were subjected to stirring for
adsorption (%) = (C0 − Ce)/C0 × 100%
(1)
q (mg/g) = V × (C0 − Ce)/m
(2)
where C0 (mg/L) and Ce (mg/L) are the initial and after sorption equilibriums U(VI) concentrations, respectively, and m (g) and V (mL) represent the mass of agar-MMT-GO and the volume of the suspension, respectively. Breakthrough Experiment. The breakthrough experiment was conducted in a glass column with a diameter of 3 cm and a height of 5504
DOI: 10.1021/acssuschemeng.7b00841 ACS Sustainable Chem. Eng. 2017, 5, 5503−5511
Research Article
ACS Sustainable Chemistry & Engineering 30 cm. The flow rate was 4 mL/min. Small fragment pieces of agarMMT-GO composites (5 g) were added to the glass column. In the first step, deionized water was used to wash the sorbent in a down-flow fashion. The main purpose of this step was to equilibrate the sorbent particles before the column test begun. Then, the pH was adjusted, and the fixed U(VI) solution was continuously fed from a reservoir at the top of the column at a desired flow rate controlled by a valve. Finally, the column effluent was intermittently collected. The concentration of U(VI) in column effluent was measured using spectrophotometrically Arsenazo III method at wavelength of 650 nm until the concentration of U(VI) in the effluent remained constant.
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RESULTS AND DISCUSSION Morphologies of Agar-MMT-GO. Figure 1a depicts the synthesis process and the schematic structure of agar-MMTGO composites. MMT was assumed to embed into the GO nanosheets, and then the mixtures were cross-linked by agar which created a porous scaffold structure. Figure 1b,c shows the SEM images of exfoliated GO and MMT nanosheets, respectively. The exfoliated GO and MMT nanosheets were essentially monolayers or multilayers from the SEM images. The average sizes of GO and MMT nanosheets were about 1.3 μm and 200 nm (data not shown), respectively, which was consistent with previous reports.18,33 The macroscopic block agar-MMT-GO composites are shown in Figure 1d,e. As shown in Figure 1e, agar-MMT-GO composites can bear the weight of 500 g, demonstrating its tenacity and toughness. The microscopic structure of agar-MMT-GO composites can be seen from their SEM images in Figure 2. Figure 2a,c exhibits
Figure 3. TEM images of agar-MMT-GO composites and corresponding element mapping.
3a. The magnified sections in the square are shown in Figure 3b. A lamellar sheet, corresponding to carbon mapping, was obviously observed in Figure 3b,c, demonstrating that GO was intactly and uniformly distributed in agar-MMT-GO composites, thus keeping high sorption performance. The basis constituent of MMT was a silica tetrahedron and alumina octahedral sheet.33 The occurrence of Si and Al mapping in Figure 3d,e suggests the structural brace effect of MMT in agarMMT-GO composites. The detected U mapping demonstrated that U(VI) could easily cross into the inner section of agarMMT-GO composites, since the TEM sample was selected from the inside of the composites (Figure 3f). The SEM and TEM results suggest that the structure of agar-MMT-GO composites is competent for heavy metal removal from solutions. FTIR and TGA Analysis. FTIR spectra of GO, MMT, and agar-MMT-GO are shown in Figure 4a. For the MMT spectrum, bands at 460 and 520 cm−1 were attributed to plane vibration of −OH functional group from an interlayer water molecule.33 The extremely strong peak at 1030 cm−1 was attributed to Si−O band stretching vibration.33 The proximate peaks at 1510 and 1680 cm−1 in the GO spectrum represented the characteristic CC and CO stretching vibrations, respectively.18 The mentioned peaks above for GO and MMT can be clearly seen in the FTIR spectrum of agarMMT-GO composites, testifying that GO and MMT were the main component of agar-MMT-GO composites. Additionally, chemical property of GO or MMT component was not changed in the composites, which indirectly proved that this 3D material was formed mainly by the force of the bracket effect from agar. TGA traces of GO sample showed ∼26.34% weight loss below ∼150 °C, which was chiefly ascribed to evaporation of the free water (Figure 4b). Then, at temperatures between ∼150 and 300 °C, a sharp weight loss of 60.13% was observed, due to the quick process of exfoliating and releasing affluent oxygen-containing functional groups on the GO slices. When temperature exceeded 300 °C, the weight loss ratio remained basically unchanged. However, there was almost no weight loss for MMT sample except little change below 150 °C (6.32%) and above 600 °C (9.18%), which was consistent with other studies.33 The TGA trace of agar-MMT-GO composites falls in between that of GO and MMT. Below 150 °C, the blue line of agar-MMT-GO composites coincided with the red MMT line, and only a tiny weight loss (6.32%) was observed. The sharp
Figure 2. Different sectional SEM images of agar-MMT-GO composites: lateral section (a, c) and vertical section (b, d).
the lateral surface structure of agar-MMT-GO composites. Honeycomb, duct-like tunnels were observed in Figure 2c, which was chiefly attributed to the porous agar or MMT getting into the middle of GO. Figure 2b,d shows the vertical section of agar-MMT-GO composites. Agar-MMT-GO composites were formed with stacking structure. The stacking structure with alternate GO and MMT sheets endowed agar-MMT-GO composites with high surface area, and the tunnels formed then allowed pollutants to easily penetrate to the inner structure of adsorbent, thus promoting U(VI) sorption to agar-MMT-GO. The satisfactory sorption performance of agar-MMT-GO was evidenced by macroscopic experiments. Figure 3 shows the TEM images of agar-MMT-GO (Figure 3a,b) and corresponding elemental mapping (Figure 3d−f). The multilayer structure of agar-MMT-GO is shown in Figure 5505
DOI: 10.1021/acssuschemeng.7b00841 ACS Sustainable Chem. Eng. 2017, 5, 5503−5511
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ACS Sustainable Chemistry & Engineering
Figure 4. FITR (a) and TGA (b) curves of GO, MMT, and agar-MMT-GO composites.
Figure 5. (a) Effect of time on U(VI) sorption on agar-MMT-GO composites, C0 = 15 mg/L, T = 303 K, pH 4.5, msorbent = 0.01 g; simulation of kinetics data by liquid film diffusion model (b) and intraparticle/pore diffusion model (c); (d) U(VI) sorption isotherms on agar-MMT-GO composites at T = 303, 313, and 323 K, pH 4.5, msorbent = 0.01 g.
decrease emerged until the temperature exceeded 300 °C. The results imply that the stability of agar-MMT-GO composites was much enhanced relative to that of GO. The improved performance of the composites will be propitious to practical large-scale application. The weight percentage (mass %) of MMT in the agar-MMT-GO composites was determined as 23% after calcining in furnace at 1000 °C. Sorption Kinetics and Isotherm. The kinetics curve of U(VI) enrichment on agar-MMT-GO is depicted in Figure 5a, and the corresponding parameters are given in Table 1. It is obvious that the sorption pattern versus time is divided into
two stages: rapid response phase and equilibrium phase. It should take 10 h for agar-MMT-GO to stride across the rapid response phase into equilibrium phase. Compared to GO in our previous reports (equilibrium time < 30 min),18 agar-MMTGO needed longer times to reach equilibrium. Two reasons might explain the slow sorption rate of agar-MMT-GO. First, U(VI) needed to enter into the interior of macroscopic bulk agar-MMT-GO composites via the channels to reach adsorption equilibrium. Second, the blocking of the pore with increasing cation also retarded U(VI) diffusion to the interior. Although the sorption rate of agar-MMT-GO was slower than that of GO, the separation and recovery of agar-MMT-GO were easier, faster, and more cost-effective, all of which are beneficial to large-scale application in wastewater remediation. Rate of sorption process was influenced by parameters including structural property of sorbent (i.e., surface area, particle size, or porosity), metal species (i.e., ionic or coordinated metal ions) and concentrations, and the affinity of metal ions with active sites on the sorbent.36 On this account, kinetics data were further simulated by liquid film diffusion model and intraparticle/pore diffusion model, which are expressed as eqs 3, and 4, respectively.37−39
Table 1. Kinetic Parameters of U(VI) Sorption on AgarMMT-GO Composites Liquid Film Diffusion Model kfd (h−1) R2
0.249 0.975
Intraparticle Diffusion Model ki (mg g−1 h1/2) C R2
17.99 98.36 0.945 5506
DOI: 10.1021/acssuschemeng.7b00841 ACS Sustainable Chem. Eng. 2017, 5, 5503−5511
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ACS Sustainable Chemistry & Engineering
Figure 6. (a) Effect of pH and ionic strength on U(VI) sorption on agar-MMT-GO composites at C0 = 15 mg/L, T = 303 K, msorbent = 0.01 g. (b) U(VI) species distribution as a function of pH calculated by Visual MINEQL+3.0 based on Nuclear Energy Agency (NEA) database; C0 = 15 mg/L, T = 303 K.
ln(1 − qt /qe) = −k fdt
(3)
qt = kit 0.5 + C
(4)
where R is the universal gas constant (8.314 J (mol K)−1); T is the temperature in Kelvin; and K0 is the sorption equilibrium constant (i.e., Kl in eq S1) at different temperatures. The values of ΔS°/R andΔH°/R can be obtained from the intercept and slope, respectively, by plotting ln(K0) versus 1/T. Negative values of ΔG° (i.e., −0.52 kJ/mol at 303 K) and positive values of ΔS° (46.45 J (mol K)−1) prove that U(VI) sorption on agar-MMT-GO was a spontaneous process. Positive values of ΔH° (8.8 kJ mol−1) give evidence of the endothermic process for U(VI) sorption on agar-MMT-GO, which suggests inner-sphere surface complexation.42 It is known that uranium is present as hydrated ions in solutions. Thus, the inner-sphere sorption of U(VI) on adsorbent has to denude its hydration sheaths, which requires energy. However, the binding of U(VI) to the surface sites of agar-MMT-GO is exothermic. When the energy of dehydration process exceeds the attachment process, ΔH° is positive, and vice versa.33 Therefore, the dehydration of hydrated U(VI) needed more energy than binding on the surface sites of agar-MMT-GO. Overall, U(VI) sorption on agar-MMT-GO composites was an endothermic and spontaneous process. Effect of pH and Ionic Strength. Figure 6a shows the pH influence on U(VI) sorption onto GO and agar-MMT-GO at 0.001 mol/L NaClO4. Sorption performance of agar-MMT-GO toward U(VI) was superior to that of GO at pH 5.5. The sorption of U(VI) on GO and agar-MMT-GO increased with increasing pH from 3.0 to 7.0. For example, the sorption percentage of agar-MMT-GO increased from 45 to 82%. A slight decrease of U(VI) sorption was observed for GO at pH >7.0, but the decreased sorption was not obvious for agar-MMT-GO. Chemisorption or electrostatic attraction more readily occurred for GO or agarMMT-GO with the increase of pH since surface sites (i.e., −OH and − COOH groups) were progressively deprotonated, accompanied by the increasing negative charge.18 The distribution of U(VI) species versus pH was calculated by Visual MINEQL+3.0 and is shown in Figure 6b. At pH >7.0, more negative charged species like (UO2)3(OH)7− and UO2(OH)3− gradually dominated. Thus, the decreased sorption at high pH can be attributed to the electrostatic repulsion between the negative U(VI) species and the negative charged adsorbents. GO was more negatively charged than was MMT with increasing pH, which might explain why the decreased degree of U(VI) sorption on agar-MMT-GO was less than that on GO at pH >7.0. The effect of ionic strength on U(VI) sorption to agarMMT-GO is also shown in Figure 6a. U(VI) sorption on agar-
where qt (mg/g) and qe (mg/g) are the sorption amount of U(VI) at equilibrium and time t, kfd (h−1) is the sorption rate constants, ki (mg g−1 h1/2) is the pore diffusion rate constant, and C is the intercept. A three-step sorption process was assumed for agar-MMTGO composites: (1) U(VI) transferred from solution to the surface of agarMMT-GO composites (film diffusion) (2) U(VI) transferred from the surface of agar-MMT-GO composites to the active sites in the interior (pore diffusion) (3) U(VI) bound with the active sites Generally, the third step is rapid, and the rate-limiting step might be the first or the second step. The fitting results of film diffusion model and pore diffusion model are shown in Figure 5b,c, respectively. The initial 10 h curve part (nonequilibrium sorption phase) can be better simulated by film diffusion model (R2 = 0.98) than by the pore diffusion model (R2 = 0.96). Both fitting plots did not pass through the origin, indicating that they are not the singular rate-limiting step. Thus, the sorption of U(VI) onto agar-MMT-GO could be controlled by both the film diffusion and the pore diffusion processes, that is, the sorption of U(VI) to agar-MMT-GO was controlled by its diffusion from the aqueous solution to the surface sites of agarMMT-GO. The sorption isotherms of U(VI) on agar-MMT-GO composites at 303, 313, and 323 K are shown in Figure 5d. The adsorption capacity increased with increasing temperature from 303 to 323 K, suggesting that U(VI) sorption on agarMMT-GO was promoted by temperature. The maximum sorption capacity of agar-MMT-GO composites toward U(VI) reached 142.8 mg/g at pH 4.5 and T = 303 K, which is comparable to the currently available data (Tables S1 and S2). On the basis of the temperature-dependent sorption isotherms, thermodynamic parameters including standard free energy (ΔG°, kJ mol−1), standard enthalpy change (ΔH°, kJ mol−1), and standard entropy change (ΔS°, J (mol K)−1) can be calculated by the following equations.40,41
ΔG° = −RT ln(K 0)
(5)
ln(K 0) = ΔS°/R − ΔH °/RT
(6) 5507
DOI: 10.1021/acssuschemeng.7b00841 ACS Sustainable Chem. Eng. 2017, 5, 5503−5511
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ACS Sustainable Chemistry & Engineering
Figure 7. Breakthrough curve of U(VI) sorption on agar-MMT-GO composites (a) and corresponding Yoon and Nelson equation modern (b), inlet U(VI) concentration = 150 or 300 mg/L, pH 4.5, msorbent = 5 g, flow rate = 4.0 mL/min; breakthrough of agar-MMT-GO composites after first regeneration (c) and sorption capacity after different regeneration cycle (d).
Table 2. Breakthrough Time and Yoon and Nelson Parameters for Adsorption of U(VI) at pH 4.5 Yoon and Nelson parameters adsorbent
inlet concn (mg/L)
breakthrough time frombreakthrough curves (min)
τ (min)
k′ (min−1)
k
agar-MMT-GO
300 600
470 200
518 272
0.055 0.057
28.52 15.05
200 min for 300 mg/L). The shape of two curves was very similar. There was no U(VI) detected in stage I, and then a steep increase of U(VI) concentration was observed in stage II. Finally, the curve entered a plateau period. The Yoon and Nelson equation was used here to depict the 50% breakthrough time of U(VI) adsorption on agar-MMTGO composites. The Yoon and Nelson equation is expressed as44
MMT-GO decreased with increasing NaClO4 concentration at pH 5.5. Outer-sphere surface complexation (i.e., cation exchange) is more sensitive to the variation in ionic strength than inner-sphere surface (i.e., covalent bonding).18 Therefore, the sorption mechanism between agar-MMT-GO and U(VI) was inferred as outer-sphere surface complexation/ cation exchange and inner-sphere surface complexation in the low and high pH regions, respectively, which was further evidenced by XPS results. On the basis of our previous research, U(VI) sorption on GO was independent of ionic strength over entire range of pH between 3.0 and 9.0.18 Thus, the enhanced sorption portion at pH
